The present invention relates to electrochemical treatment of metals and, in particular, to electrochemical machining of continuous metal webs or strips. The invention has particular application to the formation of thin metal foils from metal webs.
Typically, continuous metal foils are formed from thicker-gauge metal webs by rolling, usually cold rolling. This technique works well for metals which have good elongation and which are not work-hardenable. But for certain metals, such as titanium, the formation of foils by rolling is extremely difficult, if not impossible. Foil may also be produced by vapor deposition or plating, but this process is extremely slow and is usually associated with special alloys or elements.
Electrochemical machining of metals is a well established technology, and is used extensively to machine and deburr metal parts. Electrochemical machining is, in principle, the reverse of electroplating. Typically, in electrochemical machining a metal workpiece, which acts as an anode, is mounted for connection with the positive terminal of a DC source, and working electrode or cathode is connected to the negative terminal of the DC source and is disposed closely adjacent to the workpiece. The opposed surfaces of the cathode and the workpiece define a processing gap into which a liquid electrolyte is supplied for establishing a current path between the cathode and the workpiece for electrochemically removing material from the workpiece. A continuous flow of electrolyte fluid is maintained through the working gap to carry away material removed from the workpiece.
In order to progressively remove material from the workpiece at an even rate, a narrow working gap of a substantially constant prescribed dimension must be maintained. Prior electrochemical machining systems are all batch process type systems, wherein one or perhaps several discrete workpieces are processed in a single batch operation, after which another batch is processed in a separate operation. In such batch processes, the working gap is maintained by feeding the working or tool electrode toward the workpiece at a substantially constant speed, which depends upon the rate of removal of material from the workpiece. Such batch type systems are not suitable for continuous treatment of web or strip material, such as for reducing the thickness thereof to form metal foils.
It is known to perform other types of electrochemical processing operations on continuous web material. Thus, continuous etching or graining of metal webs is known. In such systems the metal web is moved between electrodes immersed in an etching bath. Such a system is disclosed, for example, in U.S. Pat. No. 3,779,877, issued Dec. 18, 1973. But such etching techniques are not suitable for continuous electrochemical machining of web material. More particularly, the spacing of the electrodes from the metal web is not controlled to close tolerances, as is required for electrochemical machining. Furthermore, these systems utilize an essentially static electrolyte bath, and do not provide for the flow of electrolyte fluid through a working gap, which is necessary in electrochemical machining to assure transport of removed material from the working gap. Furthermore, it is a fundamental object of such etching systems to avoid any significant reduction in the thickness of the web material, which is the exact opposite of the intent in the electrochemical machining of webs, such as in the formation of reduced-thickness foil materials.